BEIRUT, Lebanon — Far from being depleted by its recent sweep into Iraq, the extremist Islamic State in Iraq and Syria is pressing deeper into Syria, regaining territory it had lost to the mainstream Syrian insurgents just as the Syrian Army has come within five miles of encircling the insurgent-held section of Aleppo, Syria’s largest city.

The dual advance is on the verge of dealing a potentially fatal blow to the mainstream insurgents, leaving them besieged in the city while ISIS, a group deemed too extreme even by Al Qaeda, faces the Syrian government across a crucial front line at the city and surrounding province of Aleppo, the linchpin of northern Syria.

The developments, some of the most strategically important in Syria’s three-year war, come as the American military for the first time strikes ISIS fighters in Iraq. The United States says it aims to prevent massacres of Iraqi civilians, but apparently has no plans to strike the group in Syria, where it incubated into perhaps the world’s most dangerous Islamist extremist group.

That seeming contradiction highlights the messy snarl of conflicts sweeping the region: The United States now views ISIS as a global threat, a position that places America notionally on both sides of the bitter conflict in Syria, where both the insurgents and the government claim to be fighting the group.

While most analysts say there is no proof the Syrian government controls or is formally aligned with ISIS, as some rebels allege, many observers, including those sympathetic to the government, say it has not attacked the group as forcefully as it has the insurgents. That, they say, is because ISIS focuses less on ousting President Bashar al-Assad than on establishing an Islamic state in areas it controls, and battles Islamist and nationalist insurgents bent on his removal. Beheadings and other atrocities by ISIS also bolster the government’s argument that it is fighting terrorism.

Syrian insurgents in Aleppo say they are on the verge of a defeat that could effectively end their fight to unseat Mr. Assad. Rebel villages in northern Aleppo have fallen in recent days to a new ISIS offensive that threatens to cut off their supply lines to Turkey. Some insurgents have joined ISIS rather than fight it, either to save themselves from beheadings or out of frustration that their own leaders have been unable to secure weapons and money.

In Aleppo, insurgents are bracing for a siege like the two-year standoff that obliterated much of the center of Homs in central Syria, and ended in May when starved rebels accepted a government deal to evacuate. Insurgent leaders say about 500,000 civilians, mainly those too poor or sick to flee, remain in rebel-held Aleppo from a population of several million before the war, along with fighters’ families and supporters.

Struggling Western-backed insurgents express frustration that the United States has not aided them as allies against both ISIS and Mr. Assad, while some of their rank and file blame the leadership.

For three years, leaders of the rebel Free Syrian Army complain, they have pleaded for increased military aid and humanitarian corridors to protect against government airstrikes — more than 2,000 in the past three months in Aleppo alone — that have killed and displaced countless Syrian civilians. They have received only a modest response, even recently as they battle ISIS.

“Obama cannot stop ISIS by just hitting them in Iraq,” Hussam al-Marie, the Free Syrian Army spokesman for northern Syria, said in an interview from southern Turkey. He said that ISIS’ core refuges and resources are in Syria, where the group has long ruled the northeastern province of Raqqa and recently took control of neighboring Deir al-Zour.

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Aleppo after a reported barrel-bomb attack by government forces, who have retaken important territory from the rebels.CreditZein Al-Rifai/Agence France-Presse — Getty Images

“The noose is tightening around Aleppo, and everyone is just watching,” Mr. Marie said, adding that the loss of the city would be “unrecoverable” and “a blow to our shared goals of a moderate Syria.”

He recalled President Obama’s declaration that America was “coming to help,” with airlifted supplies and airstrikes, in response to the cries of Iraq’s threatened Yazidi minority.

“Millions of Syrians have been crying, but no one has heard them,” he said, adding that 8,000 Syrians were displaced by ISIS advances on Wednesday alone, and hundreds of thousands more by government attacks.

Parallel complaints can be heard from some residents in government-held Aleppo, home to a large Christian community. Some of them ask why the United States is defending Iraqi Yazidis from religiously motivated ISIS attacks, but not them.

At the same time, some Syrian insurgents blame their own leadership for what they see as impending doom.

“They failed politically,” Abu al-Hassan, a longtime spokesman for a rebel brigade in Marea, north of Aleppo, said as the town came under ISIS shelling on Thursday. “We don’t have central command, whereas ISIS has central command and an ideology.”

The crisis was a long time coming. For months, as government troops inched toward Aleppo, insurgent leaders failed to provide the province’s many rebel groups, large and small, with a unified strategy, money or weapons.

Aleppo has long stood as a symbol of both the strengths and the weaknesses of the insurgency. Rebel groups like the Tawhid Brigade surprised the government by taking over eastern Aleppo two years ago.

But the move also dealt a blow to popular support for the insurgency, bringing war into the city over the objections of many residents. Since then, government bombardments have laid waste to much of Aleppo, including parts of its renowned medieval covered market; insurgents have shelled civilians on the government-held side, and checkpoints have squeezed food and supplies to both sides.

ISIS extremists later moved in from the east, bringing harsh religious rule and killing many of the early political activists from the rebellion that began with protests against Mr. Assad. Other insurgent groups pushed ISIS out early this year, perhaps helping turn its sights toward Iraq. But now it is coming back, better armed.

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The site of a reported attack by government forces on Wednesday in Aleppo, Syria.CreditZein Al-Rifai/Agence France-Presse — Getty Images

Once ISIS takes as much territory as it can in Aleppo, a Lebanese security official monitoring the situation said, the Syrian Army will attack it, knowing that if it is hitting ISIS rather than Western-backed rebels, “The U.S. will remain silent.”

A Syrian involved in the coordination between the government and its Lebanese ally, Hezbollah, said months ago that the government had a longstanding plan for the siege of Aleppo: It would divide the rebel-held area into sectors, shell the borders between them to make them impassable, and squeeze each area one by one.

The sense of impending defeat was palpable in Aleppo Province. Two years ago, Abu al-Hassan, short and cheerful, sat comfortably in an office in Marea, a sleepy farm town, issuing reports to the international news media. The town had been left to itself, a kind of rear-guard base for the homegrown rebellion, with no ground fighting but frequently hit by government airstrikes.

But on Thursday, Abu al-Hassan was packing his belongings, about to break his vow never to leave. He pronounced himself disgusted with the rebel leaders — now blaming one another for losses to ISIS, which he said in some cases had taken villages with a very small force — and with their decision to attack ISIS months ago.

“If you want to fight the cat, you must bear its scratches,” he said. “If you can’t handle a battle with ISIS, don’t start one.”

He said he could not blame fighters for joining ISIS, which rebels said had beheaded a group of rival insurgents in a nearby town.

“ISIS fighters didn’t come from Mars,” he said. “Fifty percent of them are from here, Syrians, I know them personally. They left the F.S.A. after what they saw from corrupt commanders. Some of them said, ‘Either we leave or we die!’ ”

“Either we stop fighting ISIS or we continue and we become the regime’s ally,” he said, adding, “Goodbye, revolution!”

A version of this article appears in print on , on Page A4 of the New York edition with the headline: Dual Threat Has Mainstream Syrian Rebels Fearing Demise. Order Reprints | Today’s Paper | Subscribe